Method of forming regular array of microscopic structures on a substrate

ABSTRACT

A method of forming a substantially regular array of microscopic structures on a surface of a sample ( 4 ) is described. The sample comprises a microscopic layer of at least one first material on a substrate of a second material, wherein the microscopic layer is sufficiently thin that stress fields at the interface of the microscopic layer and the substrate cause formation of separated regions of the first material on the substrate. The microscopic layer on the sample ( 4 ) is irradiated by means of a particle beam ( 5 ) at an acute angle α, to influence the direction of alignment of said separated regions and/or the relative position of adjacent said separated regions.

[0001] The present invention relates to a method of forming a substantially regular array of microscopic structures on a substrate. The invention relates particularly, but not exclusively, to a method of forming nanometre sized wire-like structures on a substrate.

[0002] Methods of imposing patterned surface structures on articles have applications in the fabrication of various devices, for example data storage media, microelectronic and micro-electromechanical devices, sensors, optoelectronic display devices, and other optical and optoelectronic components such as optical components for directing atomic beams, as well as applications in tissue engineering and for cell adhesion or non-adhesion, or for guiding motion or growth of cells, liquids or molecules, or in molecular scale filters. Known patterning processes involve optical lithography processes or direct-write patterning techniques such as electron beam lithography and scanning probe methods.

[0003] However, optical lithography processes suffer from the drawback that they are of limited resolution, and direct-write patterning techniques, although capable of achieving higher lateral resolution than optical lithography, are impracticable for use in mass production applications because the serial nature of the surface modification process is inherently slow. Also, when applied to larger areas of material, for example more than a few square centimetres, such processes suffer poor large scale dimensional accuracy, and in the case of patterning formed by step and scan techniques, poor registration between separate write operations and scanned regions is achieved.

[0004] Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.

[0005] This provides the advantage that the surface layer from which the structures are to be formed can be formed at substantially the same time as the formation of the substantially regular array of structures, by carrying out both steps by means of the same particle beam or beams.

[0006] The method may further comprise the step of adjusting the direction of at least one said first particle beam relative to said surface layer.

[0007] This provides the advantage of enabling the nature and/or formation of the substantially regular array of structures to be adjusted.

[0008] The adjustment step preferably comprises rotating said surface layer relative to at least one said first particle beam.

[0009] The method may further comprise the step of moving at least one said first particle beam relative to said surface layer.

[0010] Said step of moving at least one said first particle beam preferably comprises scanning at least one said beam across said surface layer, or moving said surface layer relative to at least one said first particle beam.

[0011] The method may further comprise the step of mounting said substrate to an earthed support.

[0012] This provides the advantage that under certain circumstances, at least one said particle beam may impact on the earthed support, which may result in at least partial neutralisation of any electrostatic charge building up on the surface layer or on the substrate.

[0013] The method may further comprise the step of stabilising said substantially regular array of structures.

[0014]FIG. 3 is a schematic view of an apparatus for carrying out a process of a second embodiment of the present invention.

[0015] Referring to FIG. 1, a surface patterning apparatus 1 for forming nanometre sized structures of calcium or calcium-rich material on the surface of a substrate of calcium fluoride comprises a vacuum chamber 2 containing a particle beam source 3 for irradiating a sample 4. The particle beam source 3 directs a particle beam 5 at an angle α onto sample 4, which is mounted to a support 6 such as an earthed aluminium support. The support 6 can be rotated by means of a suitable device 7 such as a motor, the purpose of which will be described in greater detail below.

[0016] The operation of the apparatus 1 shown in FIG. 1 will now be described by means of the following example.

EXAMPLE

[0017] A sample 4 of calcium fluoride was produced by cleaving a calcium fluoride crystal in air at room temperature and pressure, by striking the crystal with a sharp instrument along a direction that generally coincided with the intersection of a (111) crystal plane with one of the crystal surfaces. The freshly cleaved sample 4 was then placed in the vacuum chamber 2 which was evacuated to a pressure of approximately 4×10⁻⁸mbar, and the major residual gas components of which were hydrogen and water vapour. Introduction of the sample 4 into the vacuum chamber 2 was assisted by the use for sample introduction of an intermediate small vacuum chamber (not shown) that could be rapidly evacuated to a pressure of approximately 10⁻³mbar.

[0018] The sample 4 was mounted to a support 6 comprising an electrically earthed aluminium plate, and was irradiated under an angle of incidence α of approximately 10° to the sample surface by a beam 5 of singly positively charged argon ions having a kinetic energy per incident ion of approximately 4,500 electron volts (eV). The sample 4 was held fixed relative to The sample 4 as shown in FIG. 2 was removed from the vacuum chamber 2 and examined by means of a commercially available atomic force microscope, equipped with ultra sharp silicon probe tips supplied by NT-MDT Co. Moscow, having a nominal tip end radius of less than 10 nm. Examination of the sample 4 revealed arrays of linear structures which were always found to be generally parallel to the projection of the axis of the ion beam 5 on the sample surface 4, and to be approximately two atoms in height and approximately 10 nm in separation.

[0019] The method described with reference to FIG. 1 can under certain circumstances also be used to form multiple layers of such structures by means of sequentially carrying out the method of FIG. 1 described above. For example, the formation of superimposed layers of wire-like structures (arranged directly or indirectly on top of each other) generally at right angles to each other could be used to construct high density memory devices.

[0020] Referring now to FIG. 3, in which parts common to the embodiment of FIG. 1 are denoted by like reference numerals but increased by 100, a surface patterning apparatus 101 for carrying out a method of a second embodiment of the invention also includes within vacuum chamber 102 a second beam source 108, being a source of atoms, molecules or clusters which are deposited on the surface of sample 104 by means of a second beam 109. This allows material to be deposited on the surface of sample 104 at the same time as, or independently of, irradiation of the sample 104 by means of the first ion beam 105.

[0021] In the case of the apparatus 101 of FIG. 3 being used in connection with materials where the stress field between the surface layer and the substrate would tend to cause the formation of compact, non-elongated regions of the material forming the surface layer, for example semiconducting materials, by rotating the sample 104 relative to the ion beam 

1. A method of forming a substantially regular array of structures on a substrate, the method comprising: providing a surface layer of a first material on a substrate of a second material, wherein said surface layer is sufficiently thin that stress fields at the interface of said surface layer and said substrate cause formation of separated regions of said first material on said substrate; and directing at least one first particle beam onto said surface layer and at a respective acute angle thereto to influence the direction of alignment of said separated regions and/or the relative position of adjacent said separated regions.
 2. A method according to claim 1, wherein the step of providing said surface layer comprises depositing said layer on said substrate.
 3. A method according to claim 2, wherein said surface layer is deposited by means of at least one second particle beam.
 4. A method according to any one of the preceding claims, wherein the step of providing said surface layer comprises modifying the surface of said substrate by means of at least one particle beam.
 5. A method according to claim 4, wherein the surface of said substrate is modified by means of at least one said first particle beam.
 6. A method according to any one of the preceding claims, further comprising the step of adjusting the direction of at least one said first particle beam relative to said surface layer. least partially filling at least some gaps comprises adsorbing at least one gaseous material into said gaps.
 16. A method according to claim 14, wherein the step of at least partially filling at least some gaps comprises depositing material into said gaps.
 17. A method according to any one of the preceding claims, including directing at least two said first particle beams onto said surface layer, wherein at least two said first particle beams are not parallel to each other.
 18. A method according to any one of the preceding claims, further comprising the step of forming a plurality of said substantially regular arrays of structures.
 19. A method according to claim 18, wherein at least two said substantially regular arrays of structures are arranged in separate layers.
 20. A method of forming a substantially regular array of structures on a substrate, the method substantially as hereinbefore described with reference to the accompanying drawings. 